In general, cerium oxide is a highly functional ceramic material that is widely used in catalysts, phosphors, cosmetics and abrasives. Recently, cerium oxide has come into the spotlight as an inorganic abrasive material essential to form advanced CMP (chemical mechanical polishing) slurry for use in a shallow trench isolation process in the field of semiconductor devices.
Important characteristics of CMP slurry for STI CMP processes include a polishing rate to a silicon oxide (SiO2) layer, polishing selectivity between a silicon oxide layer and a silicon nitride (Si3N4) layer, and an anti-scratching property to a surface to be polished. To provide slurry having such important characteristics, it is required to control the particle size, shape, oxidation behavior, crystallinity, strength, etc. of cerium oxide powder used as an abrasive material. Cerium oxide-based slurry shows lower dishing and erosion as compared to conventional silica-based slurry, and has high polishing selectivity between a silicon oxide layer and a silicon nitride layer. Thus, there has been a tendency to substitute silica-based slurry with cerium oxide-based slurry in various CMP processes.
Although cerium oxide has a lower strength than the strength of a silicon oxide layer and a silicon nitride layer to be polished, it still can be used as an abrasive material. This is because polishing of a silicon oxide layer is made not only by a mechanical polishing action but also by a chemical reaction caused by cerium oxide. This is supported by a recent report on chemical reactions of cerium oxide (Journal of Non-Crystalline Solids, 283 (2001) pp. 129-136).
Meanwhile, conventional polishing slurry has required additives in addition to the polishing slurry in order to increase the selective polishing rate ratio (referred to also as ‘selectivity’ hereinafter) of silicon oxide layer/silicon nitride layer to a level of 20:1 or higher. Such additives mainly comprise an excessive amount of polycarboxylic acid-based anionic polymers. However, addition of such additives has a problem in that the additives deteriorate dispersibility of abrasive particles when they are mixed with the polishing slurry, so that the abrasive particles precipitates severely and has a non-uniform concentration in the slurry. In brief, conventional polishing slurry containing additives shows non-uniform polishing characteristics. The aforementioned problem has been solved to date by providing polishing slurry essential to perform polishing and additive slurry for realizing a high selectivity, separately (i.e. providing a two-component composition), and by mixing the two kinds of slurries right before carrying out polishing.
In another way, one-component cerium oxide-based polishing slurry has also been suggested despite the aforementioned severe precipitation phenomenon, the one-component polishing slurry being obtained by preliminarily mixing additives for realizing a high selectivity with the cerium oxide-based polishing slurry. However, in this case, it is not possible to obtain uniform polishing characteristics due to the aforementioned severe precipitation phenomenon.